Add a switch which controls whether multicontexts veto modification events from their subcontexts.
Add a simple context manager to control deferring events on a DataContext.
Modernize context code

  * use absolute imports
  * use new traits adaptation mechanisms
  * remove a few old-style classes
  * assume with statement exists
Bump the version number.
Modernize context mask code.
Modernise execution code.
Modernize and clean-up blocks code

Includes removing copy of uuid module, since we no longer require 2.4 support
Fix executing context for new adaptation mechanism.
Add classes and interfaces to execution api.
Move defer notification to a method on IListenables.
Move to use (Python) context managers and with statement where appropriate.
Test const_assign works.
Use adapt() instead of interface adaptation.
Remove some other used of interface constructore adaptation.
Modernization of some old utility modules.
Modernization.
Update docs.
Add a changelog.
Make interface match implementation :P
Fix events_deferred method calls.